Star Trail

Star Trail

While staying in Sussex (fantastic hospitality from David and Clare) I took the opportunity to experiment with star trail images. It’s quite straightforward to do – you just need to do things in the right order.

You need…

  • Camera (with an interval timer – you can use a separate interval timer if you have one)
  • Tripod
  • Wide angle lens
  • Star stacking software

Find somewhere dark! A little light may not be too bad, but you’ll need to make sure there’s not too much light from other sources as it could ruin your exposure.

Set your camera up on your tripod, with wide angle lens. Point at a suitable place in the sky. I normally try to include the Pole star, as you will catch the other stars as they rotate around it. Include some of the surrounding landscape if you like – whatever makes your image.

Set your lens to it’s widest aperture and focus on infinity. Be careful, as some lenses can focus past infinity (which, as I write, sounds bonkers, but I’m told this is so).

While you are learning – set your image quality to jpeg. You can always change to suit yourself, but smaller images mean less processing time on the computer.

Set your ISO to the highest you are comfortable with. Try ISO 1600, but you may need to go higher – see below.

Now here’s the technical bit. Take the focal length of your lens, and divide it into 600. The number you end up with is the maximum amount of seconds your shutter can be open before the stars blur. For example, if you have an 18mm lens, then 600/18=33.3. So your shutter should be open for no longer than 33 seconds. In astrophotography this is called ‘the rule of 600’. There is an assumption here – the calculation is based on cameras with a full frame sensor (such as Nikon D810). For APS-C (or crop frame) sensor cameras, you will need to calculate by adding 50% to the focal length of your lens.  For example, if you have an APS-C camera and a lens of 10mm focal length, you add 50% to that focal length, giving you 15mm. Use that number for your calculation using the rule of 600.

Set your camera to Manual exposure and Manual focus – you do not want those to change. Now take a test image and review on your cameras LCD monitor.

If your exposure is as you want, and you can clearly see the stars, then you’re good to go. If too dark, then adjust the ISO upwards and retest. Repeat until you are happy with the exposure. If your image is too light, then reduce the ISO and retest. Again, repeat until you are happy with the exposure.

Now set up your intervalometer for the number of images you want to take, and the interval between exposures. Remember to make sure your interval is greater than the exposure time you have set. For the image above I took 100 images of 25 seconds exposure every 30 seconds (giving me 5 seconds between exposures), but you can set to however many images you wish to. Experiment.

Handy tip – whilst not such an issue in the dark, when you take long exposure images, stray light can enter via the viewfinder. Cover it off with anything you have handy. I have, on occasions, hung a baseball cap over the back of my camera – it does the trick.

Now set your intervalometer going and leave your camera to take the images.

When all the images have been taken there’s one more thing you have to do. Put the lens cap on and take a final (dark) exposure. This will be needed later.

Nearly there! Load the images on the computer and put them into a folder Don’t forget the dark exposure image. Load up your star stack application of choice – I use StarStax for Mac, but there are plenty of other applications out there. Load up your images, load up the dark exposure, and process. Save the resulting file.

And that’s all there is to it! 🙂

Go out and experiment.